1. Field of the Invention
This invention relates in its most general aspects to a containment structure for enclosing any dangerous material for any device which may create a dangerous environment upon failure. The invention is particularly directed to the containment of nuclear power plant reactors wherein a failure of the reactor coolant system could result in the release of dangerous radioactive gases, liquids, or vapors as postulated for certain hypothetical accidents including that referred to as the "maximum credible accident", which postulates the complete and very rapid release of the contents of the reactor coolant system into the interior of the containment structure.
2. Description of the Prior Art
Containment structures for nuclear reactors have been developed to the point where they are a standard component of all nuclear power plants. Some leakage from containment structures built in accordance with the prior art can be tolerated for nuclear power plants sited in low population density areas. However, in the case wherein the nuclear reactor is located in close proximity to more heavily inhabited areas, a reliable, fail-safe containment structure is essential. In densely populated areas it is vital that the containment structure be capable of preventing the leakage of all radioactive gases, liquids, or vapors to the atmosphere for even the shortest period of time.
The trend in the nuclear power industry is toward locating nuclear reactors in close proximity to more heavily inhabited areas. Economic and environmental reasons dictate that a minimum length of transmission lines be used to convey the electrical power generated by the nuclear reactor to the ultimate user. In addition, as populated areas expand and encroach on otherwise suitable power station sites, containment designs must be improved to compensate for shorter exclusion distances.
Considerable work has been done in developing containment structures for nuclear reactors. Among the various structures developed for containment of nuclear reactors are double walled containment structures such as disclosed in U.S. Pat. No. 3,322,141 (Gans et al., May 30, 1967), U.S. Pat. No. 3,320,969 (Gordon, May 23, 1967) and U.S. Pat. No. 3,258,403 (Malay, June 28, 1966). However, in each of these structures the containment pressure is always positive with respect to the atmosphere. Even with these double walled nuclear reactor containment structures, the out-leakage of vapors and gases from the inner boundary will continue indefinitely after a nuclear reactor coolant system failure until the containment can be safely vented to atmospheric pressure.
The concept of the previously known double walled containment structures is also disadvantageous in that it required all outleakage from the inner containment boundary and all inleakage from the outer containment boundary to be pumped back into the interior containment, thereby slowly building up pressure in the interior containment until such time as the interior containment could be safely vented to the atmosphere. This is not a practical or desirable solution in the long term from a safety standpoint since the contained energy of the containment structure becomes increasingly higher and no subsequent, long-term failures in the containment barrier system can be tolerated. In addition, in the double wall containment structure designs previously developed, access to the containment interior for decontamination and subsequent maintenance is impossible until such time as the pressure in the containment interior can be reduced to near atmospheric pressure. This access requirement becomes especially important for it prevents the repair of small breaks which result in less than maximum credible accident conditions and radiation exposures that can be tolerated for practicable periods of time.
Vacuum or subatmospheric containment structures having a single essentially impervious membrane or liner have also been previously developed. However, even with vacuum or subatmospheric systems, the containment pressure is positive with respect to the atmospheric pressure for a period of time, on the order of one hour following a maximum credible accident. A typical subatmospheric containment pressure transient for a maximum credible accident is shown in FIG. 2. Thus, a pressure which is initially about 5 psi below atmosphere rises in about 20 seconds to about 37 psi above atmospheric pressure, or about 52 psia. At this time the containment pressure reduction safety features become operative and the containment pressure is reduced to less than atmospheric pressure in about an hour.
During this approximate one hour time period, liquid and gaseous radioactivity can leak through the single containment boundary. Dependent on circumstances such as the exclusion distance (distance of the structure from the site boundary), and the meteorological conditions, the outleakage during this time of positive pressure operation may require pre-treatment with additional engineered safety systems, such as alkaline containment spray or containment air recirculation systems with particulate and activated charcoal filters, which only partially remove radioactive materials.
Although it has also been previously suggested that vacuum or subatmospheric containment structures may be designed such that the interior of the containment does not reach atmospheric pressure in the event of an accident, and also that double containment may be utilized in conjunction with vacuum or substmospheric operation of a containment structure, no structure for implementing these suggestions has been presented and, furthermore, these suggestions do not encompass the reliable, failsafe containment system of the present invention.